The generation and detection of ultrasonic acoustic waves is widely used for imaging the interior of solid objects in nondestructive testing and evaluation (NDE), as well as for dimension gauging and for measuring material properties such as elastic constants. The most common devices used for generation and detection of ultrasound are piezoelectric transducers, which typically have very high sensitivity. These transducers require direct contact with the object being tested or indirect contact through a liquid column or a solid wedge. However, in many applications completely noncontact ultrasonic inspection, where there is no physical contact, either directly or indirectly, with the object being tested, is desirable. Examples include inspection of objects which are at high temperature, objects with curved surfaces, objects sensitive to contamination, or where conditions require that fast scans be preformed.
Ultrasonic inspection can be performed without direct or indirect physical contact using a pulsed laser that generates ultrasound at the surface of the object to be inspected, which then propagates to the object's interior, in combination with optical interferometric apparatus and methods that detect the very small undulations created by reflected waves reaching the surface of the object. This technique is generally called laser-ultrasonics (LUT) or laser-based-ultrasound (LBU). A description of several applications of LUT can be found in "Nondestructive evaluation with laser ultrasound," Mechanical Engineering, Vol. 116, p.63, 1994. However, because of the small amplitude of typical ultrasonic displacements compared to the light wavelength, optical detection remains a challenge. Thus, various techniques have been investigated and developed for optical detection of ultrasound, as described, for example, by J. P. Monchalin, "Optical detection of ultrasound," IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. UFFC-33, p. 485, 1986, and by J. W. Wagner, "Breaking the sensitivity barrier: the challenge for laser ultrasonics," IEEE Ultrasonics Symposium, 1051-0117/92, p. 791, 1992.
Laser-ultrasonics instrumentation for use in the field requires an interferometric system that is lightweight, compact, and environmentally rugged. Conventional interferometers do not possess these characteristics because they require the exact alignment of optical components on a rigid and heavy optical bench. Assembling the interferometer with optical fibers eliminates optical alignment problems and the need of an optical bench. However, perturbations such as temperature changes and vibrations cause a substantial change of the fiber delay and consequently most fiber-optic interferometers are adversely susceptible to perturbations of this type.
The J. E. Bowers and G. S. Kino, U.S. Pat. No. 4,572,949, Feb. 25, 1986, discloses prior art of a fiber-optic ultrasonic detector that avoids the problems caused by temperature changes and vibrations by using a configuration known in the art as a Sagnac interferometer. A detailed explanation of its working principle is also found in the corresponding patent document. FIG. 6, adapted from the referred patent, shows this prior art. Unfortunately, in the Sagnac interferometer, one-half of the light reaching the detector does not convey a signal useful for detecting desired information, and therefore, that light increases the background noise and limits the instrument sensitivity. Moreover, the response obtained by the Sagnac interferometer is proportional to the square of the ultrasonic signal amplitude, which is very small, unless a phase modulator is inserted inside the loop. Inserting the phase modulator creates harmonic distortion and consumes electrical power, and in addition the modulator is expensive. An alternative embodiment disclosed in the referred patent is to use a polarization controller inside the Sagnac loop and take advantage of the fiber residual birefringence, but with this approach the advantages of environmental insensitivity are lost because the interferometer is sensitive to perturbations to the fiber birefringence.